An electrically driven power steering apparatus includes a steering torque sensor detecting steering input torque supplied via a steering wheel by driver's operation and a control device determining a magnitude and a direction of motor output based on an output signal of the steering torque sensor. An electric motor is driven via an inverter circuit on the basis of the determination of the control device. Motor power is transmitted to a steering system, so that steering torque is reduced.
In conventional electrically driven power steering apparatuses, a switch composed of a relay is inserted between a battery serving as a power source and an inverter circuit. When detecting an overcurrent state or a defect of PWM control, a control device opens the switch to cut off power supply to the inverter circuit and the motor, thereby preventing generation of undesired assisting steering force by the motor. However, the switch configured of the relay is required to supply to the motor a large current ranging from several tens A to 100 A in order that steering assistance torque may be generated. A relay which can open and close a path along which the large current flows is large-sized. In view of the problem, a switch circuit has been proposed for use instead of the relay. The proposed switch circuit uses a semiconductor switching element such as a field-effect transistor (FET).
FIG. 6 shows an example of configuration conceived in the above-described case. An inverter circuit 1 includes six power metal-oxide semiconductor field-effect transistors (power MOSFETs) 2U to 2W and 2X to 2Z (N channel) configured into a three-phase bridge arrangement. The motor 3 includes three-phase windings (not shown) connected to three-phase output terminals of the inverter circuit. The motor 3 is a brushless DC motor, for example. A vehicle battery 4 has a positive terminal connected via a switch circuit 5 to a positive DC bus bar of the inverter circuit and a negative terminal (body earth) to a negative DC bus bar.
The switch circuit 5 includes two N-channel MOSFETs 6a and 6b connected via a common source to each other. The N-channel MOSFET 6a has a drain connected to a positive terminal of the battery 4, and the N-channel MOSFET 6b has a drain connected to a positive DC bus bar of the inverter circuit 1. Both N-channel
MOSFETs have respective gates connected in common with a resistive element 7 which is further connected between the gates and sources of the MOSFETs.
A drive circuit 8 driving the switch circuit 5 is configured as a peripheral circuit such as a micro control unit (MCU; and microcomputer) which is an IC controlling the inverter circuit 1. Electrical power is supplied from the battery 4 via a diode 15. A circuit ground is connected to a negative DC bus bar of the inverter circuit 1. A power generation circuit 9 generating driving power to drive the switch circuit 5 has an output terminal. A series circuit of two N-channel MOSFETs 10 and 11 is connected between the output terminal of the power generation circuit 9 and the ground. These FETs 10 and 11 have respective gates. A drive signal supplied from the MCU is further supplied via a half-bridge (H/B) drive circuit 12 to the gates of the FETs 10 and 11 individually. Protecting diodes 13 and 14 are connected in parallel to the N-channel MOSFETs 10 and 11 respectively.
The MOSFETs 10 and 11 have common connection points (source and drain) connected to the gates of the N-channel MOSFETs 6a and 6b respectively. When the switch circuit 5 is turned on according to a drive signal from the MCU, the H/B drive circuit 12 turns on the N-channel MOSFET 10 and turns off the N-channel MOSFET 11, thereby turning gate potentials of the N-channel MOSFETs 6a and 6b to a high level. Furthermore, when the switch circuit 5 is turned off, the N-channel MOSFET 10 is turned off and the N-channel MOSFET 11 is turned on, so that the gate potentials of the N-channel MOSFETs 6a and 6b are turned to a low level.
Thus, with respect to a device mounted on a vehicle and supplied with power from the battery 4, it needs to be considered whether or not the circuit is protected when the battery 4 is connected in the reverse direction. Assume now the case where the battery 4 is connected in the reverse direction in the configuration shown in FIG. 6. The following problem arises in this case. As shown in FIG. 7, since the electric potential at the negative DC bus bar of the inverter circuit 1 rises, voltage is applied via the following path:
the positive terminal of the battery 4—the negative DC bus bar—the diode 14 (or the body diode of FET 11)—the switch circuit 5 (the gate-drain of FET 6a)—the negative terminal of the battery 4.
As a result, potential difference exceeding a threshold is applied to the path between the gate and the source of the FET 6 of the switch circuit 5, so that the switch circuit 5 is turned on thereby to cause electric current to flow along the above-mentioned voltage application path. Simultaneously, when the FET 6b side is turned on, current also flows through body diodes of the FETs 2U and 2X composing the inverter circuit 1. In this case, there is a possibility that the elements constituting the path may be broken by a short-circuit current flowing through the battery 4.
It is considered that the switch circuit 5 is connected with a P-channel MOSFET 16 being used as one of switching elements so that parasitic diodes have a common anode, as shown in FIG. 8. This connecting manner does not result in a problem of the above-described reverse flow. However, the P-channel MOSFET has a larger element size as compared with an N-channel MOSFET. Furthermore, the FETs need to be supplied with gate signals with different levels for purpose of control. Accordingly, the switch circuit is generally configured of two N-channel FETs. This necessitates the overcoming of the above-described problem.